The present invention relates to methods for the synthesis of piperazine rings, particularly for the preparation of heterocyclic compounds useful as intermediates in the synthesis of piperazinoazepines such as the antidepressant mirtazapine.
Mirtazapine, also known as 1,2,3,4,10,14b-hexahydro-2-methylpyrazine [2,1-a]pyrido[2,3-c]benzazepine, is an antidepressant suitable for oral administration. It has a tetracyclic chemical structure unrelated to other classes of antidepressants such as selective serotonin reuptake inhibitors (SSRIs), tricyclics or monoamine oxidase inhibitors. Mirtazapine belongs to the piperazinoazepine group of compounds, and has the following structural formula. 
Known methods for the preparation of piperazine ring derivatives have low yields (9-30%), expensive reagents, and many reaction steps (Roderick, W. R. et al., J. Med. Chem 9, 1966, 181-185). It is desirable to have methods for preparing piperazine ring derivatives with fewer steps, high yields and inexpensive raw materials.
The present invention relates to a novel process for preparing a compound of the formula I: 
wherein
R1 denotes substituted or unsubstituted alkyl, aryl, arylalkoxy, tosyl, formyl, benzoyl, acetyl or amine; R2 denotes substituted or unsubstituted alkyl, alkoxy, aryl, aryloxy or arylalkoxy; and R3 denotes substituted or unsubstituted alkyl, alkoxy, aryl, aryloxy or arylalkoxy; by reacting a compound of the formula 
wherein R2 and R3 are as defined above and R4 and R5 are independently selected from the group consisting of fluoro, chloro, bromo and iodo,
with a compound of the formula H2Nxe2x80x94R1, wherein R1 is as defined above. Preferably the reaction is performed in the presence of a solvent. Polar aprotic solvents such as, dimethyl formamide, dimethylacetamide and dimethylsulfoxide are preferred.
The present invention provides a new process for preparing piperazine rings suitable for use in the synthesis of the antidepressant mirtazapine and other tetracyclic compounds such as those disclosed in U.S. Pat. No. 4,062,848 to van der Burg, the contents of which are incorporated herein by reference.
The process of the present invention comprises the steps of reacting a compound of formula II: 
wherein
R4 and R5 are independently any of the of radicals selected from the group that consists of fluoro, chloro, bromo and iodo; and R2 and R3 are as defined above; with a compound of the formula H2Nxe2x80x94R1, wherein R1 is as defined above. Preferably, R1 denotes aryl, acetyl, formyl, benzoyl, amine, or tosyl. Most preferably, R1 is tosyl. In order to remove any doubt, the tosyl radical is defined as the group of formula VI: 
wherein Me represents a methyl group. Preferably R2 denotes methyl, R3 denotes phenyl, R4 denotes chloro, and R5 denotes chloro.
Preferably, the compounds of formulae I and II are compounds of formulae IV and V accordingly: 
In a preferred embodiment, the present invention relates to a process for preparing a compound of formula XI: 
wherein Ph represents a phenyl group, which comprises reacting a compound of formula XII: 
with a compound of formula XIII: 
Preferably this reaction takes place in the presence of a strong base such as sodium hydroxide (NaOH), sodium hydride (NaH), potassium hydroxide (KOH), potassium hydride (KH), sodium methoxide (NaOMe) and sodium amide (NaNH2). Sodium hydroxide and sodium hydride are preferred.
Preferred solvents for the above reaction are any one or more of the solvents selected from the group that consisting of dimethyl formamide (DMF), dimethyl acetamide (DMAC), dimethyl sulfoxide (DMSO), xylene, benzene, ethylbenzene, acetonitrile, toluene and ethers with high boiling points such as ethyleneglycol dimethyl ether, diethyleneglycol dimethyl ether, and propyleneglycol dimethyl ether.
The compound of formula XI may be further hydrolized to give the compound of formula XIV: 
The compound of formula XIV is known as 4-methyl-2-phenylpiperazine. Compounds of formula XI may be hydrolyzed by reacting a compound of the formula XI with acid to give compounds of the formula XIV. Preferred acids for the reaction are strong acids such as sulfuric acid (H2SO4), hydrochloric acid (HCl), phosphoric acid (H3PO4) and p-toluene sulfonic acid. A more preferred acid is sulfuric acid with a concentration of about 98%. Preferably the reaction is carried out in aqueous solution.
The compound of formula XIV may be used in the preparation of the mirtazapine (1,2,3,4,10,14b-hexahydro-2-methyl-pyrazino[2,1-a]pyrido[2,3-c][2]benzazepine), as shown in Schemes 1 and 2 below. 
As shown in Scheme 1, the compound 3-cyano-2-(4-methyl-2-phenyl-1-piperazynyl)pyridine may be prepared starting from 2-chloro-3-cyano-pyridine and 4-methyl-2-phenyl-piperazine.
Starting from 3-cyano-2-(4-methyl-2-phenyl-1-piperazynyl)pyridine, mirtazapine can be prepared by two routes, which are further presented in Scheme 2: 
In accordance with the present invention, mirtazapine produced by the process of the present invention may be prepared as pharmaceutical compositions that are particularly useful for the treatment of depression. Such compositions comprise mirtazapine with pharmaceutically acceptable carriers and/or excipients known to one of skill in the art.